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Bill Stone is a maverick cave explorer who has plumbed Earth's deepest abysses. In this talk, he explains what it's like to descend into the deepest caves in complete darkness for days on end — and why he keeps doing it.

About Bill Stone

Engineer and daredevil caver Bill Stone pushes the frontier — through flooded tunnels, the remotest depths of the Earth and the limits of human endurance. He's currently planning to mine moon ice by 2015 and build an autonomous robot to visit the icebound sea of Jupiter's moon, Europa.

His Shackleton Energy Company intends to raise enough money to mine ice thought to be trapped on the moon's southern pole at Shackleton Crater, and to sell derived products on the moon and in low Earth orbit to international consumers.

It's the TED Radio Hour from NPR. I'm Guy Raz. And today on the show, we're talking to people who've gone all the way to the edge, and we're asking them why? Back when Bill Stone was in high school, he decided he wanted to go to the moon, which, as we'll hear later, he still does. But anyway, one day, as he was walking to class, he saw a flyer was pasted on the wall, and it was for something called The Spelunking Club.

BILL STONE: The Spelunking Club. And I said, I don't know what that word means. And I got my mother to take me over to the meeting, and suddenly, here was this thing about exploration. There was an image of a person on rope rappelling into a black abyss. It was such a perfect shot, because the pure utter blackness of what he was rappelling into, the starkness of him against that, the sun shining down on this moss-lit shaft. It was just like Jules Verne right there before my eyes, and that was it, that was the scene. It got me hooked right away.

RAZ: And everything changed at that moment. Cave exploring actually became his profession, a life right at the edge of human possibility. What you do is climb Everest in reverse. I mean, these caves are as deep as Everest is high.

STONE: That's true but with one critical difference. On Everest, they're shedding weight as they're coming down and they're coming down light. We don't have that option. When we're at the bottom of a cave, you actually have to pull your way back out. So the hard part starts at the bottom.

RAZ: Here's Bill's TED Talk.

(SOUNDBITE OF TED TALK)

STONE: One of the things that's changed here in the last 150 years since Jules Verne had great science fiction concepts of what the underworld was like is that technology has enabled us to go to these places that were previously completely unknown and speculated about. We can now descend thousands of meters into the earth with relative impunity. Along the way, we've discovered fantastic abysses and chambers so large that you can see for hundreds of meters without a break in the line of sight. When you go on a thing like this, we can usually be in the field for anywhere from two to four months with a team of as small as 20 or 30 to as big as 150.

RAZ: What does it feel like to be inside those caves?

STONE: Yeah, it's sensory deprivation is what it is. Imagine going into a world that consists nothing but blacks and browns and some occasional tans and maybe, if you're really lucky, you know, a dash of white here and there by, you know, calcite in a vein of a rock or something like that. So it's monotonous color accompanied by the low rumble or hiss of water. Ubiquitous, it's just kind of rattling around sonically through all these chambers, because the river is always there. You can be a level or two above it or you can be right into it. If you're into it, it's loud.

It's loud enough that it's shaking the walls as if you were standing beside a locomotive. You can physically feel the acoustic, you know, resonates. And so the longer you spend down there, the more you get acclimated to that sensory deprived environment, such that when you come back to the surface, there's a period of about 15 to 20 minutes where you're in sensory overload. Colors that are hot colors just leap out, almost in 3D. You can hear things like buzzing insects from farther away. And the other thing is you can usually smell the entrance from within anywhere from 500 meters to 800 meters inside, just because the wind is carrying that subtle scent down into the cave. It's this heightened state of sense until you reach the point of saturation.

RAZ: Wow, that's crazy.

STONE: Yeah, yeah.

(SOUNDBITE OF TED TALK)

STONE: We have already gone far beyond the limits of human endurance from the entrance. This is nothing like a commercial cave. You're looking at Camp 2 in a place called J2, not K2, but J2. We're roughly two days from the entrance at that point. And the idea is to try to provide some measure of physical comfort while you're down there, otherwise in damp, moist, cold conditions and utterly dark places. I should mention that everything you're seeing here, by the way, is artificially illuminated at great effort. Otherwise, it is completely dark in these places.

The deeper you go, the more you run into a conflict with water. It's basically like a tree collecting water coming down. And eventually, you get to places where it is formidable and dangerous. If there is a monster underground, it's the crushing psychological remoteness that begins to hit every member of the team once you cross about three days inbound from the nearest entrance.

STONE: The deeper you get the more the presence of what they have come down plays on their mind. And they get into this little cyclonic spiral. And when you can tell that they've got this, and a good old friend of mine, he called this rapture of the deep, you know, kind of a play on Jacques Cousteau.

And it's true. You can see these people. They will typically sit there and wrap their arms around their knees and just sit there. And they kind of like have dilated pupils, more so dilated than you would if you're just down there in the dark. And you can tell they're obsessed with, are they going to be able to get out?

RAZ: Why do you think you're immune to the rapture?

STONE: Oh, I'm not immune to it. It's a matter of degree. When I first started doing expeditionary work in southern Mexico, none of us had ever camped underground. When we got down there, I was doing the same thing. I was thinking, boy, we just came down a lot of drops, and that bag was really heavy, and we were having gravity assisting us on the way down. Am I going to be able to get out?

Well, you know, that was on my mind for the first couple days, and then, of course, the excitement of being the first to explore that place, you know, pretty much took over. And then at the end it was basically like, OK, if I can get up one drop, I can get up the next. If I can get the next, I can get up the one after that. And then it's a matter of routine, just getting used to it. But you still feel this every time you go beyond a personal experience limit.

RAZ: There are dozens of ways you can die in a cave.

STONE: Hell, there're dozens of ways you can die bicycle trafficking.

RAZ: Yeah, but I mean, when you're in a cave, you're far away from a rescue crew. I mean, the people who work with you know the risks. What are the risks that they're made aware of?

STONE: Typically, the two that people worry about are moving water and rock fall. Rocks that have sat there for millennia can be sitting right precariously teetered such that you can walk onto a piece of rock that's the size of a gigantic truck, and the thing will just rock over a couple of degrees.

I've had this happen so many times that, you know, it's kind of a joke. They called them the rocking rock, or sometimes they call it the widow maker and other things like that. So every once in a while, every once in a while, they'll roll away from you.

(SOUNDBITE OF TED TALK)

STONE: Next year, I'll be leading an international team to J2. We're going to be shooting from minus 2,600 meters. That's a little over 8,600 feet down at 30 kilometers from the entrance. The lead crews will be underground for pushing 30 days straight. I don't think there's been a mission like that in a long time. Eventually, if you keep going down in these things...

RAZ: You gave this talk in 2007. You have now been to J2, to this cave that many people believe or, I guess it's been proved, is the deepest cave on earth. What did you find?

STONE: This is a question that many Mexican local residents on these mountains always ask. They think you're bringing out gold. Otherwise, why would you do this? There has to be some financial payoff, you know. The answer is the satisfaction that comes out of this whole thing is knowledge. It's hard to explain that to somebody who hasn't been there, but when you get done, three and a half, four months' worth of work will typically lead to a couple of squiggles added to a three-dimensional computer map of this cave system within the mountain.

And so it's part of the idea of just playing this game out, and it's you against the mountain, mano-a-mano. And there's a period of about four to five weeks that we tend to refer to as post-expedition blues. And it's like you're sitting back at your desk going, this is really pretty lame and useless, and, you know, compared to what I was doing. I was doing something, you know. I was out there on the edge, you know, stepping in the ground where nobody had ever been before, you know, exploring. And you can't compete with it. You absolutely cannot compete with it in day-to-day life, sitting in a cubicle at a computer.

RAZ: How important is it for you to get to the deepest place on earth, to touch that spot?

STONE: I'm not obsessed with it. It's more of a thing where, as you see these things come together, these are gigantic puzzles. They're three-dimensional chess games inside a mountain. And the game is played out over decades. This is the rationalization that has come. But why do it? I'll tell you why. It's because it is the real frontier on earth. It is the last terrestrial frontier for humans. Now you could say, wow, what about the oceans? Well, what about the oceans?

We know what's down there. What we don't know, for example, is where the deepest point you can get inside the earth is. That is still, just like in Jules Verne's age, an unknown, open question. And there are people now armed with technology that never existed before that is allowing us to do things that would have been pure science fiction just 40, 50 years ago.

RAZ: Bill Stone, caver, explorer and, one day, moon dweller. Bill's company is now trying to lead a mining expedition to the moon's southern pole. He's hoping to find fuel there to power future space stations. Are you still planning to do that? Do you think that you will plant the flag there on the moon?

STONE: The short answer is yes. And we are closer today than we ever were, even in 2007 when we made that statement. Things take time. We have agreements, in fact, with all NASA centers regarding technology transfer. So my prediction is that probably within the next year it's going to happen. Transcript provided by NPR, Copyright NPR.